1. Background of the Invention
The present invention relates to a sintering method for a tungsten-nickel-manganese (hereinafter, called "W--Ni--Mn") type heavy alloy, and in particular, to an improved sintering method for fabricating a W--Ni--Mn type heavy alloy which has no pores and a 100% theoretical density.
2. Description of the Conventional Art
A W--Ni--Mn type heavy alloy is composed of 90 weight % tungsten, and more than 0.5 weight % manganese and nickel.
The W--Ni--Mn type heavy alloy is composed of almost globular tungsten particles and a matrix phase of a W--Ni--Mn type heavy alloy into which a part of the tungsten is melted as shown in FIG. 1.
In a W--Ni--Mn type heavy alloy, an adiabatic shear band is maximized by adding manganese the thermal conductivity of which is low, instead of the iron(Fe) and copper(Cu) contained in a conventional W--Ni--Fe type and W--Ni--Cu type heavy alloys, and the heavy alloy can suitably be used as a material for a penetrant of a kinetic energy penetrator in the munitions field.(A Belhadjhamida and R. M. German, "The Effects of Atmosphere, Temperature, and Composition on the Densification and properties of W--Ni--Mn," compiled by J. M. Capus and R. M. German, VOL. 3, MPIF, Princeton, N.J., 1992, pp 47-55.)
A correlation between the penetrating force of a kinetic energy penetrator and an adiabatic shear band can be explained according to the schematic diagrams in FIGS. 2 and 3.(L. S. Magness and T. G. Farrand, "Deformation Behavior and its Relationship to the Penetration Performance of High Density Penetration Materials", Proc. 1990 Army Science Conf., Durham, N.C., May 1990, pp 149-164.) As shown in FIG. 2, when a penetrator of a material on which an adiabatic shear band does not occur collides with an object as shown in FIG. 2, it is easily transformed into a mushroom shape(,which is called "mushrooming") and the kinetic energy is dispersed across a relatively broad region. On the other hand, as shown in FIG. 3, when an adiabatic shear band occurs easily, the kinetic energy is concentrated into a narrow region. Since such a difference in energy concentration degree relates to the penetration force, the development of a penetrant material in which an adiabatic shear band occurs easily is of essential interest.
Factors which influence an adiabatic shear band are the specific heat, the strain hardening exponent, the thermal softening, the melting point and the thermal conductivity. The most important factor of all is known to be the thermal conductivity, since an adiabatic shear band is closely related to the heat transfer phenomenon.(A. Bose, H. Couque, J. Lankford, Jr., "Influence of Microstructure on Shear Localization in Tungsten Heavy Alloys," ed. by A. Bose and R. J. Dowding, Proc. Tungsten and Tungsten Alloys, MPIF, Princeton, N.J., 1992, pp 291-298.) Therefore, recently much attention has been focused on a W--Ni--Mn heavy alloy containing manganese the thermal conductivity of which is extremely low, and in which the adiabatic shear band occurs easily.
W--Ni--Mn type heavy alloys are fabricated by means of a powder metallurgy, as in W--Ni--Fe type and W--Ni--Cu type heavy alloys. Such a conventional sintering method as shown in FIG. 4 will now be described in detail.
As shown in FIG. 4, conventionally, W--Ni--Mn type heavy alloys are fabricated by a liquid phase sintering under a hydrogen atmosphere.
That is, the reason for keeping the heavy alloy at 800.degree. C. for 60 minutes during the sintering process is for the purpose of deoxidizing the oxides of tungsten, nickel and manganese existing on the surface of the material powders of W--Ni--Mn type heavy alloys under a hydrogen atmosphere.
However, according to the thermodynamic data concerning oxidation/deoxidation of each element, while tungsten and nickel are easily deoxidized at the above temperature range, but manganese is not deoxidized, but rather the oxides are set in a more stable condition. This means that the oxygen separated in the deoxidation of tungsten and nickel reacts with the manganese, resulting in a formation of manganese oxide. Since this manganese oxide becomes stable thermodynamically and is not easily deoxidized, residual porosity is disadvantageously formed during the sintering, as in the fine microstructure as observed through a SEM shown in FIG. 5.
These residual pores lower the mechanical strength of W--Ni--Mn type heavy alloys and consequently limit their use as a penetration material for a kinetic energy penetrator.
Therefore, to utilize W--Ni--Mn type heavy alloys as a kinetic energy penetrator, the formation of pores should be minimized. Therefor, studies have been conducted using a VHF (vacuum hot press) method instead of a liquid phase sintering, and studies are being conducted to reduce porosity through such a process as HIP (hot isostatic pressing) or by a thermal mechanical treatment carried out after performing a liquid phase sintering. But, in spite of such efforts, the reality is that alloys of greater than 98% relative theoretical density have not been obtained and the VHP or HIP process performed after a liquid phase sintering are undesirably costly.